diff options
Diffstat (limited to 'Documentation/driver-api/usb/dma.rst')
| -rw-r--r-- | Documentation/driver-api/usb/dma.rst | 54 |
1 files changed, 14 insertions, 40 deletions
diff --git a/Documentation/driver-api/usb/dma.rst b/Documentation/driver-api/usb/dma.rst index 2b3dbd3265b4..02f6825ff830 100644 --- a/Documentation/driver-api/usb/dma.rst +++ b/Documentation/driver-api/usb/dma.rst @@ -10,7 +10,7 @@ API overview The big picture is that USB drivers can continue to ignore most DMA issues, though they still must provide DMA-ready buffers (see -:doc:`/core-api/dma-api-howto`). That's how they've worked through +Documentation/core-api/dma-api-howto.rst). That's how they've worked through the 2.4 (and earlier) kernels, or they can now be DMA-aware. DMA-aware usb drivers: @@ -60,7 +60,7 @@ and effects like cache-trashing can impose subtle penalties. force a consistent memory access ordering by using memory barriers. It's not using a streaming DMA mapping, so it's good for small transfers on systems where the I/O would otherwise thrash an IOMMU mapping. (See - :doc:`/core-api/dma-api-howto` for definitions of "coherent" and + Documentation/core-api/dma-api-howto.rst for definitions of "coherent" and "streaming" DMA mappings.) Asking for 1/Nth of a page (as well as asking for N pages) is reasonably @@ -91,46 +91,20 @@ Working with existing buffers Existing buffers aren't usable for DMA without first being mapped into the DMA address space of the device. However, most buffers passed to your driver can safely be used with such DMA mapping. (See the first section -of :doc:`/core-api/dma-api-howto`, titled "What memory is DMA-able?") +of Documentation/core-api/dma-api-howto.rst, titled "What memory is DMA-able?") -- When you're using scatterlists, you can map everything at once. On some - systems, this kicks in an IOMMU and turns the scatterlists into single - DMA transactions:: +- When you have the scatterlists which have been mapped for the USB controller, + you could use the new ``usb_sg_*()`` calls, which would turn scatterlist + into URBs:: - int usb_buffer_map_sg (struct usb_device *dev, unsigned pipe, - struct scatterlist *sg, int nents); + int usb_sg_init(struct usb_sg_request *io, struct usb_device *dev, + unsigned pipe, unsigned period, struct scatterlist *sg, + int nents, size_t length, gfp_t mem_flags); - void usb_buffer_dmasync_sg (struct usb_device *dev, unsigned pipe, - struct scatterlist *sg, int n_hw_ents); + void usb_sg_wait(struct usb_sg_request *io); - void usb_buffer_unmap_sg (struct usb_device *dev, unsigned pipe, - struct scatterlist *sg, int n_hw_ents); + void usb_sg_cancel(struct usb_sg_request *io); - It's probably easier to use the new ``usb_sg_*()`` calls, which do the DMA - mapping and apply other tweaks to make scatterlist i/o be fast. - -- Some drivers may prefer to work with the model that they're mapping large - buffers, synchronizing their safe re-use. (If there's no re-use, then let - usbcore do the map/unmap.) Large periodic transfers make good examples - here, since it's cheaper to just synchronize the buffer than to unmap it - each time an urb completes and then re-map it on during resubmission. - - These calls all work with initialized urbs: ``urb->dev``, ``urb->pipe``, - ``urb->transfer_buffer``, and ``urb->transfer_buffer_length`` must all be - valid when these calls are used (``urb->setup_packet`` must be valid too - if urb is a control request):: - - struct urb *usb_buffer_map (struct urb *urb); - - void usb_buffer_dmasync (struct urb *urb); - - void usb_buffer_unmap (struct urb *urb); - - The calls manage ``urb->transfer_dma`` for you, and set - ``URB_NO_TRANSFER_DMA_MAP`` so that usbcore won't map or unmap the buffer. - They cannot be used for setup_packet buffers in control requests. - -Note that several of those interfaces are currently commented out, since -they don't have current users. See the source code. Other than the dmasync -calls (where the underlying DMA primitives have changed), most of them can -easily be commented back in if you want to use them. + When the USB controller doesn't support DMA, the ``usb_sg_init()`` would try + to submit URBs in PIO way as long as the page in scatterlists is not in the + Highmem, which could be very rare in modern architectures. |